The invention relates to a process of the utilization of tar-dust which is a waste product separated from the gases derived from gasification under pressure of granular fuel, especially of bituminous coal, whereby the waste tar-dust is fed back into the pressure gasification cycle of the fuel for cracking of the tar and additional heat values. The tar-dust waste product is a very viscous product resembling shoe polish in its appearance.
In a known process (DL-PS 74071) separated hydrocarbons, especially tar and tar-dust, are heated and introduced into the reaction or conversion zone of a generator for the gasification of granular fuel. Here the hydrocarbons are broken down in the presence of hydrogen. The process has the advantage that the tar arising during the process of pressure gasification is included. Thus the gas is enriched by the low boiling hydrocarbons obtained from the tar. For this reason, the gas leaves the generator with a higher calorific value. Of course, the introduction of the tar into the generator causes considerable difficulties. Among other things, the tar, which is generally highly viscous, has to be forced through pipes into the generator with the aid of pumps.
Another problem with the operating of generators for pressure gasification arises from the dust content of the generated gas. Its dust content is increased with the increase of undersize granules (i.e., of 0-2 mm grain size) in the fine coal which is fed to the generator. Up to 70% of the coal obtained by the usual extraction and preparation methods is likely to be undersize. For this reason, the fine coal must be graded before it is fed to the generator so that the coal fed to the generator has generally a grain size of 2-30 mm. Use of coal having a lower grain size than this leads to an undesirable high dust content in the gas generated. Furthermore, it leads to a poor draught in the standing pillar of fuel in the body of the generator.
The tar-dust has previously been laboriously pumped and fed to the top of the gasifying unit in an attempt to crack the tar. Not only are the pipe lines often blocked, but the pumping efficiency is poor. However, gasification in this case is occurring above the top of the coal column and no cracking takes place. In fact, the net effect of this recycling is a big increase in the tar-dust mixture to be removed with no increase in heat values.
It has also been suggested to pump the tar-dust into a lower stage in the gasifier. Of course, pumping problems are still present. However, no considerable cracking of tar is obtained by this method for the tar is condensed in the upper and colder coal seams so that again the tar is recycled with no cracking.